Starter motor assemblies that assist in starting engines, such as engines in vehicles, are well known. A conventional starter motor assembly is shown in FIG. 15. The starter motor assembly 200 of FIG. 23 includes a solenoid 210, an electric motor 202, and a drive mechanism 204. The solenoid 210 includes a coil 212 that is energized by a battery upon the closing of an ignition switch. When the solenoid coil 212 is energized, a plunger 216 moves in a linear direction, causing a shift lever 205 to pivot, and forcing a pinion gear 206 into engagement with a ring gear of a vehicle engine (not shown). When the plunger 216 reaches a plunger stop, electrical contacts are closed connecting the electric motor 202 to the battery. The energized electric motor 202 then rotates and provides an output torque to the drive mechanism 204. The drive mechanism 204 transmits the torque of the electric motor through various drive components to the pinion gear 206 which is engaged with the ring gear of the vehicle engine. Accordingly, rotation of the electric motor 202 and pinion 206 results in cranking of the engine until the engine starts.
Many starter motor assemblies, such as the starter motor assembly 200 of FIG. 15 are configured with a “soft-start” starter motor engagement system. The intent of a soft start starter motor engagement system is to mesh the pinion gear of the starter into the engine ring gear before full electrical power is applied to the starter motor. If the pinion ring gear abuts into the ring gear during this engagement, the motor provides a small torque to turn the pinion gear and allow it to properly mesh into the ring gear before high current is applied. The configuration of the solenoid, shift yoke, electrical contacts, and motor drive are such that high current is not applied to the motor before the gears are properly meshed. Accordingly, milling of the pinion gear and the ring gear is prevented in a starter motor with a soft-start engagement system.
Starters with a soft start engagement system, such as that of FIG. 15, typically include a solenoid with two distinct coils. The first coil is a pull-in coil 212 and the second coil is a hold in coil 214. As shown in FIG. 15, the pull-in coil 212 is wound first on the spool 220. On top of this winding the hold-in coil 214 is wound. Sometimes this order is reversed such that the hold-in coil 214 is wound first on the spool 220 followed by the pull-in coil 212.
During operation of the starter, the closing of the ignition switch (typically upon the operator turning a key) energizes both the pull-in coil 212 and the hold-in coil 214. Current flowing through the pull-in coil 212 at this time also reaches the electric motor 202, applying some limited power to the electric motor, and resulting in some low torque turning of the pinion. Energization of the pull-in coil 212 and hold-in coil 214 moves a solenoid shaft (also referred to herein as the “plunger”) in an axial direction. The axial movement of the solenoid plunger moves the shift lever 205 and biases the pinion gear 206 toward engagement with the engine ring gear. Once the solenoid plunger reaches the plunger stop, a set of electrical contacts is closed, thereby delivering full power to the electrical motor. Closing of the electrical contacts effectively short circuits the pull-in coil 212, eliminating unwanted heat generated by the pull-in coil. However, with the pull-in coil is shorted, the hold-in coil 214 provides sufficient electromagnetic force to hold the plunger in place and maintain the electrical contacts in a closed position, thus allowing the delivery of full power to continue to the electric motor 202. The fully powered electric motor 202 drives the pinion gear 206, resulting in rotation of the engine ring gear, and thereby cranking the vehicle engine.
After the engine fires (i.e., vehicle start), the operator of the vehicle opens the ignition switch. The electrical circuit of the starter motor assembly is configured such that opening of the ignition switch causes current to flow through the hold-in coil and the pull-in coil in opposite directions. The pull-in coil 212 and the hold-in coil 214 are configured such that the electromagnetic forces of the two coils 212, 214 cancel each other upon opening of the ignition switch, and a return spring forces the plunger 216 back to its original un-energized position. As a result, the electrical contacts that connected the electric motor 202 to the source of electrical power are opened, and the electric motor is de-energized.
In order to produce a high performing vehicle starter with a soft start motor engagement system, such as that described above, designers are faced with numerous design challenges. First, the pull-in coil must be properly designed to avoid various issues that may arise during operation of the starter. As described above, when the pull-in coil of a soft-start starter motor engagement system is energized (i.e., when the ignition switch contacts close due to operator turning engine switch key on), the pull-in coil provides electromagnetic force to pull the plunger toward the plunger stop and to the closed position. However, the pull-in coil is connected electrically in series with the starter motor, and should only have a low resistance. With low resistance through the pull-in coil, sufficient current flows through the pull-in coil and to the electric motor such that the electric motor can deliver a sufficient output torque to rotate the pinion gear and avoid abutment with the ring gear, as described previously. This required torque is typically 8-12 N-m. For a 12V motor, the resistance may be on the order of 0.030 ohms so that several hundred amps flow through the motor, and also the series connected pull-in coil, during soft start. However, this low of resistance of the pull-in coil creates other design challenges. First, if the soft start period is prolonged, or repetitive starts are performed, a high amount of ohmic heat is generated in the pull-in coil because of the large amount of current flowing through the pull-in coil. For a 12V system this can be on the order of 3-4 kW, and this can lead to thermal failure of the insulation system of the wiring that forms the coils. Second, the large current through the pull-in coil creates a much stronger electromagnetic force on the plunger during closure than is needed. This may become a problem when an abutment between the pinion gear and ring gear occurs, and the impact force of the pinion gear on the ring gear can exceed 4500N. As a result, the ring gear could fracture or chip. Over time and thousands of starts, the surface of the ring gear may deteriorate and require replacement for proper starting.
Design challenges related to the pull-in coil, such as those discussed in the preceding paragraph result in additional design challenges with respect to other components of the starter, such as the hold-in coil. For example, as discussed in the previous paragraph, the pull-in coil has specific design limitations related to the current flowing through the pull-in coil. Since the electromagnetic excitation is the product of coil turns times current, and since current is fixed, this generally leaves the number of turns of the pull-in coil as the primary design variable for the pull-in coil. While the number of turns of the pull-in coil can be reduced to reduce the impact abutment force issue described previously, this presents a problem with the hold-in coil. In particular, the number of turns in the hold-in coil should match the pull-in coil so that during disengagement of the pinion gear and the ring gear following vehicle start, the electromagnetic forces of the two coils will cancel each other and allow the pinion gear to pull cleanly out of the ring gear. However, before vehicle start, the hold-in coil stays energized for a much longer period of time than the pull-in coil. Therefore, the hold-in coil should not be of low resistance or it will thermally fail. Thus, the resistance of the hold-in coil generally is an order of magnitude higher than that of the pull-in coil. The high resistance of the hold-in coil means that current flow through the hold-coil before start is relatively low, resulting in a relatively low amp-turn product. If the number of turns of the hold-in coil is too low, then the hold-in coil will deliver an insufficient magnetic force to hold the plunger closed and the starter motor will disengage before vehicle start.
As explained in the previous paragraphs, designers of vehicle starters with soft start motor engagement systems are faced with opposing design challenges for two coils that should produce equivalent electromagnetic forces. On the one hand designers strive to limit the turns of the pull-in coil in order to reduce the impact force during engagement of the pinion gear and the ring gear. On the other hand designers strive to increase the turns of the hold-in coil such that the hold-in coil delivers sufficient electromagnetic force to maintain the plunger in a closed position during engine cranking. Accordingly, it would be desirable to provide a solenoid for a vehicle starter with a pull-in coil that limits the impact force during engagement of the pinion gear and the ring gear. It would also be desirable to provide a hold-in coil for the solenoid that delivers sufficient electromagnetic force to maintain the plunger in a closed position during engine cranking. Additionally, it would be desirable if such a solenoid were relatively simple in design and inexpensive to implement.